In the March 2010 issue of Nature Geoscience, Yin and Berger analyze the warming of interglacial (e.g. present-day) intervals across the Mid-Brunhes Event (MBE) at approximately 430,000 years ago. Interglacials prior to that time were cooler, and exhibited lower sea levels, larger ice sheets, and lower atmospheric CO2, unlike the present-day interglacial or any after the MBE. In an attempt to explain why this change occurred, Yin and Berger modeled interglacial peaks over the past 800,000 years as a function of greenhouse gases and Milankovitch orbital forcing.

The Milankovitch hypothesis proposes that the 100,000 year periodicity of glacial and interglacial intervals present in the geological record is a function of orbital forcing. Changes in the Earth’s axial tilt (obliquity), rotational wobble (precession), and distance from the Sun (eccentricity) combine to affect the latitudinal distribution of incoming solar radiation across the Northern and Southern Hemispheres (i.e. boreal and austral), which is responsible for long-term climatic variation. Axial tilt and precession are the primary controls on insolation (41,000- and 25,700-year periodicity, respectively), but are amplified and modulated by longer-scale changes in orbital eccentricity (100,000 years and, to a lesser degree, 413,000 years). With this in mind, it’s clear that the discrepancies between pre- and post-MBE interglacial intervals need to be explained.

To answer the question, Yin and Berger chose standard insolation values for each successive interglacial peak across the past 800,000 years. Phase congruency between precession and tilt created the insolation standard, and interglacial peaks (i.e. the warmest part of the 100,000 year cycle) were chosen based on the marine δ18O record, otherwise named the ” δ18O minima”. Recall that the marine record exhibits lower δ18O during warm periods while glacial ice records higher δ18O due to evaporation fractionation.

What does this mean? The following figure provides a quick idea of what’s going on:

Fig.1. Obliquity, precession, and δ18O modeled over the past two interglacials from 135,000 to present day. The δ18O black bar represents the peak warming of each interglacial (δ18O minima), while the precession black bar represents when the Earth was closest to the Sun in the Northern Hemisphere summer (perihelion) and the obliquity black bar indicates the maximum tilt of the Earth (~24.5 degrees). Full article text shows 800,000-year record. Adapted from Yin and Berger (2010).

As indicated, the in-phase obliquity and precession curves indicate peak insolation, and pre-date the warmest part of the interglacial interval by about 5,000 years, which is more or less consistent with what is expected. Deglaciation is a long and protracted process with many starts, stops, and feedbacks, so it is typical for there to be a lag between maximum insolation and peak warmth. In a general sense, the Milankovitch hypothesis would only be in trouble if maximum insolation post-dated the warmest part of interglaciation.

In a snapshot discussion, Yin and Berger reveal that complications arise for specific interglacial intervals. And would we expect anything less from Mother Nature? However, averaging the pre- and post-MBE interglacials, the authors identified a clear interglacial warming after 430,000 years ago.

It appears the primary culprit is increased atmospheric CO2 post-MBE present in the geological record, in conjunction with increased winter insolation in the Northern Hemisphere. The authors cite a 60%-30% split (final 10%??), and conclude that a) boreal winters are generally warmer during interglacials after 430,000 years ago than they were pre-MBE, and that b) increased winter warming exerts a stronger control on climate than increased summer warming.

All told, the paper is a good look into some interesting aspects of the late Quaternary icehouse climate. If you have access to Nature Geoscience, check it out.

References

Yin, Q.Z., and Berger, A., 2010. Insolation and CO2 contribution to the interglacial climate before and after the Mid-Brunhes Event. Nature Geoscience, vol. 3, pg. 243-246.

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New research published in the April 2010 issue of Nature by Rosing et al. (introduced by Kasting) casts a new light on the faint young Sun paradox. Previously, scientists invoked extreme concentrations of greenhouse gases in the Archean atmosphere to account for the presence of liquid water on the Earth during a time when the Sun was less bright than it is today. This new study, however, suggests that lower albedo was the primary driver for an ice-free, early Earth.

What is the faint young Sun paradox?

Temperature-wise, liquid water exists on the surface of the Earth within very narrow window, but geological evidence indicates the presence of liquid water spanning 4 billion years. Given that the Sun was 25-30% less luminous 3-4 billion years ago, the Earth would have been frozen over with ice during the Archean under present-day conditions.

The long-standing solution to this paradox is to assume that conditions in the Archean were much different than they are today, and given what we know about the Precambrian world, it’s a reasonable assumption to make. Within this context of a changing Earth, the idea is that higher concentrations of greenhouse gases (i.e. CO2, CH4, and NH4) allowed liquid water to persist during this period of lower insolation.

Problems with the GHG solution

Creating a scenario where increased tectonic activity on the early Earth raised atmospheric CO2 levels and led to warmer temperatures is easy to accept. The problem, however, is that evidence such as Archean paleosols, weathering rinds on fluvial clasts, and evaporites point to lower atmospheric CO2 than is required to keep the early Earth ice-free.

Additionally, the presence of iron minerals like magnetite and siderite within Precambrian banded iron formations places a constraint on atmospheric CO2 to an upper limit of three times the present-day level (i.e. ~900 ppm). Minimally, this assumes a coupling (if not equilibrium) between the atmosphere and ocean in which banded iron formations were deposited. As a consequence, Rosing et al. suggest that greenhouse gas concentrations in the Archean atmosphere were too low to keep liquid water stable at that time. Increased methane as a forcing mechanism is also discounted along similar lines of reasoning.

Planetary albedo as a solution?

The authors assert that in the present day, vegetation and cloud cover limit the albedo difference between land and ocean with respect to the land/ocean difference on the early Earth. We understand that land plants did not appear until Ordovician-Silurian time (approximately 460-440 Ma), so the Precambrian is certainly different when it comes to the influence of vegetation on reducing albedo.

In terms of cloud cover, it is argued that in the present day, increased biotic input from plants and eukaryotic algae creates more cloud condensation nuclei and leads to smaller water droplets in clouds (~12 um). Smaller droplets means more scattering of incoming solar radiation, and thus, higher albedo. During the Precambrian when this biotic influence was absent, the authors suggest that cloud droplets were larger (~20-30 um) on account of fewer nucleation sites, leading to lower albedo. As we understand it, planetary albedo is inversely proportional to the average surface temperature of the Earth.

Given that there is an albedo difference between land and water, continental growth through time clearly exerts a strong control on planetary albedo. As indicated in Figure 1a, continental landmass has increased in time, even if debate over the specifics remains. According to Rosing et al., continent formation was initiated by 4 Ga, experienced rapid growth throughout the Archean and Paleoproterozoic from 3.5 to 1.5 Ga, and leveled off at approximately 1 Ga. In turn, planetary albedo also increased throughout this interval (Fig. 1b.). Figure 1c. accounts for greenhouse gas and cloud influence creating an overall increase in planetary albedo throughout geological time.

Fig. 1. a. Land/ocean ratio through time (i.e. 3.8 Ga to present day) under the assumption of continental growth. b. Estimated surface albedo where increased land leads to an increase in albedo to the present day. c. Average planetary albedo including greenhouse gas and cloud effects, indicating an overall increase. Adapted from Rosing et al. (2010).

All of this comes together in Figure 2, where the authors’ model indicates that albedo forcing at 900 ppm CO2 and CH4 with 20-30 um cloud droplets is effective in ensuring the occurrence of liquid water on the Earth as far back as 3.8 Ga.

You decide. It’s clear that significant problems arise when invoking extreme concentrations of greenhouse gases. However, as this paper illustrates, it’s not unreasonable to estimate higher concentrations of greenhouse gases than the present atmospheric level. Because I’m not well-read on the faint young Sun literature, and given that, as Kasting notes, the paradox is almost 40 years old, I wonder how this hypothesis diverges – if at all – from the standing literature. I’m just guessing, but surely this idea has been on the radar before? In any case, as far as I can tell, the authors have provided a pretty elegant response to the faint young Sun paradox.

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The Science and Technology Committee appointed by the UK House of Commons released their report on the recent Climategate scandal a couple of days ago (HTML – PDF). It’s quite an interesting read of what happens when politics and science collide, and is highly relevant to those involved with science in any capacity – especially controversial science.

If you’re hazy on what the scandal was about in the first place, the report provides a nice introduction:

“On Friday 20 November 2009 it was reported across the world that hackers had targeted a ‘leading climate research unit’ and that e-mails from the University of East Anglia’s (UEA) Climatic Research Unit (CRU), one of the world’s foremost centres of climate science, had been published in the internet. The story of the substantial file of private e-mails, documents and data that had been leaked helped ignite the global warming debate in the run up to the Copenhagen climate change conference in December 2009. As reported by the press, exchanges on the internet alleged that data had been manipulated or deleted, in order to support evidence on global warming.”

Of course, at no point was there unequivocal evidence that such tampering had occurred, nor did the actions of a scant few have any effect on the science overall, but that was a minor aside in the circus that followed.

The committee’s report comes to three main conclusions:

1. On the issue of access to information, the negative attention paid to the Climate Research Unit and its head, Phil Jones, is misplaced.

2. Unlike what was trumpeted in the media, there was no scientific dishonesty on the part of Jones or the CRU.

3. Because it’s such an important topic, climate science bears a great(er) responsibility to be “irreproachable”.

Even though the first two conclusions will largely be ignored in the coming days, they do clear up the scandal rather handily. What concerns me, however, is the third one. I think the recommendation to be “irreproachable” with published science is a nice ideal, but it’s just that – an ideal. I’m not convinced that science, especially climate science, can ever be irreproachable insofar as I understand the definition of the word. This kind of language is reserved for religionists and ideologues who do not have to contend with, and embrace the underlying uncertainty that characterizes the chaotic systems scientists investigate.